Add Slides builder, data source selector, fix cross-year labels

Features:
- Slides page: build presentations with configurable charts per slide
- Export slides as HTML zip
- Data source dropdown (Museums active, Coffees/eCommerce placeholders)
- Renamed app to 'HiHala Data' with 'Museums' as subtitle

Fixes:
- Cross-year period labels now show 'Nov 25–Jan 26' instead of misleading year
- Period display boxes updated to use smart labels

UI:
- New nav link for Slides
- Mobile nav updated
- Data source select styled to match brand

package.json

@@ -10,6 +10,7 @@
     "@testing-library/user-event": "^13.5.0",
     "chart.js": "^4.5.1",
     "chartjs-plugin-datalabels": "^2.2.0",
+    "jszip": "^3.10.1",
     "react": "^19.2.4",
     "react-chartjs-2": "^5.3.1",
     "react-dom": "^19.2.4",